Electro-static charge is a stationary electric charge which can accumulate on various surfaces. An electro-static discharge (ESD) occurs when the electro-static charge becomes substantial enough to overcome the dielectric material between any charge and another surface of lower electrical potential.
Electro-static discharge in the field of electronics can be devastating to microelectronic devices. A sharp, inadvertent voltage spike caused by an electro-static discharge can rupture the thin gate oxide of field effect transistors (FET's), or degrade a transistor's p-n junctions, effectively destroying proper integrated circuit (IC) operation.
The two most common sources of ESD stress are user handling of IC packages and machine handling of IC packages. The human body can be modeled as a 150 pF capacitor capable of storing approximately 2.0 kV, connected in series to a resistance of approximately 1.5 k.OMEGA.. When the pins of the IC package are touched, a peak current equal to approximately 1.5 amperes is passed through a device which makes up the IC. With IC's having metal oxide semiconductor (MOS) devices having 1 .mu.m or smaller geometries, discharges of approximately 1.5 amperes may damage or even destroy the gate oxides of the MOS devices if adequate ESD protection is not provided.
A machine can be modeled in the same way as the human body, except a resistance of 0 ohms is assumed. In practical applications, machine resistance is on the order of 20-40 .OMEGA.. Under the machine model, a device that can withstand approximately 400 volts of electro-static charge is considered acceptable by industry standards.
A third method of modeling ESD stress caused by machine handling of devices, is referred to as the charged device model. The charged device model is used to simulate the ESD failure mechanism associated with machine handling during the packaging and testing of semiconductor devices. According to this model, an IC package is charged to a potential of between 100V-2000V. Then, the device is discharged to ground via another device pin. The charging is normally done via the substrate pin while discharging is initiated by touching a device pin with a grounded inductance probe.
The charged device model simulates an ESD event during machine handling of packaged semiconductor devices. ESD damage from machine handling is becoming more significant than ESD damage from human handling because, while attention has been focused on minimizing human ESD damage, relatively little work has been done on minimizing the effects of ESD damage caused by machines.
One current method of preventing ESD damage is to insert a diode, a field effect transistor, or bipolar transistor between the device to be protected and a connecting pin to divert the ESD spike.
A drawback with using conventional bipolar transistors is that the bipolar transistor must first be turned on to provide ESD protection. Thus, a device has no ESD protection during its manufacturing stage.
Another drawback with conventional ESD protection devices and methods is that they do not provide protection for electrostatic charge present on the surface of an IC device.
Yet another drawback with conventional ESD protection devices and methods is that they only protect integrated circuit inputs, not the actual devices fabricated on an integrated circuit. As device features become smaller and smaller, the gate oxide thickness of devices is reduced and oxide breakdown becomes more likely in non-operating devices.
Thus, there is a need to provide ESD protection for semiconductor devices in both low power, and non-operating states.